Gene families are frequently important in cellular development and mutations may contribute to genetic diseases. It is hypothesized that some types of gene families are far more common than others in eukaryotic genomes. The immediate objective will be to determine the size and frequency of multigene families of various types in model eukaryotic organisms such as Caenorhabditis elegans. These initial results will be generalized to rat and human. Detailed analyses will focus on modestly differentiated dispersed gene families. It is hypothesized that the most common mechanism driving the development of these families is gene conversion. Evidence for such gene conversion and other historical events will be sought in several such gene families. It is hypothesized that the introns, which are frequently found, associated with the individual genes in multigene families will be especially helpful in understanding family history. Moreover, because they provide additional opportunities for homologous recombination, may play a significant role in the development of the families they are associated with over time. It is hypothesized that the presence of introns may facilitate the analysis of gene family history. Homologous introns will be identified and analyzed.